036 INTRODUCTION. 
The heart, enclosed -within a pericardium, is situated 
near the middle of the pulmonary cavity. The auricle 
and ventricle are pyrifonn, and placed base to base. 
The sides of the latter are considerably thicker than 
those of the former, and present internally several well- 
marked fasciculi, crossing in different directions. Be- 
tween the auricle and ventricle is a double valve. From 
the apex of the venti-icle passes off the aorta, which 
pierces the muscular peritoneum, and divides into two 
principal branches, — one passing to the sub-oesophageal 
ganglia devoted to the viscera in the anterior part of the 
visceral cavity, the other passing to supply the viscera 
posteriorly. Upon the right of the heart, attached to 
the roof of the pulmonary cavity, is placed a large, glan- 
dular organ, considered as the kidney ; from the -whole of 
its right margin proceeds a duct backwards, which then 
es to the side of the rectum, at the left side of which 
ic remains attached to the pulmonary orifice. 
Arion. The pulmonary cavity is situated as in 
Limax. Its -whole interior surface presents an intricate 
rete, from which converge six or seven pulmonary reins 
to the auricle of the heart. 
The renal organ forms a complete circle around the 
heart, and is perforated by the rectum, in the course of 
the latter to the pulmonary aperture. 
Tebexxophorus. The pulmonary cavity is situated 
beneath the anterior portion of the mantle. It is formed 
